Argentiferous-copper turbogenerator rotor winding



July 5, 1949. E. A. DAVIS ARGENTIFEROUS-COPPER TURBOGENERATOR ROTORWINDING Filed April 50, 1947 INVENTOR I [van A. Davis.

wnmzssas; W wM ATTORNEY Patented July 5, 1949 UNITED STAT ES PATENTQFF-I C E ARGENTIFEROUS-COPPER TURBOGEN- ERATOR ROTOR WINDING.

Evan A. Davis; Forest Hills, Pa., assignor to Westinghouse ElectricCorporation, East Pittsburgh, REA, a corporation. of Pennsylvania.

Application April 30, 1947, Serial No. 745,021.

(Cl. 1-'7-l-252) 4 Claims. 1

My invention relates: to a special copper alloy, having minute traces ofsilver in it, incertain definite proportions, for use as-the"conductor-material for the rotor-windings in turbine generators.

ivlylnvention is an improvement over the invention which is describedand claimed in the Rose Patent 2,103,795, granted December 28, 1937, inwhich a small amount of cold-working was applied to at least thecoil-sideswhichlie in the rotor-slots, for the purpose of increasing thelife of the rotor-winding from a matter of three to five years, more orless; to a matter of twenty years, more or less.

The difficulty which was being combated' arises from the fact that aturbine generator may be started and stopped a hundred times or athousand times during its life; Each time the generator is started, thecentrifugal force holds the rotor coil-sides in frictionalengagementwith the slot-wedges, thus preventing the copper from expanding, and ifthe copper is soft, the copper is usually stressed beyond itsyield-point, at the normal operating-temperatures, thus causing thecopper to sufier a permanent set. Each time the generator is stopped,the centrifugal force which has been holdingv the rotor coil-sides-infrictional engagement with the slot-wedges is released while the windingis hot, so that, when the winding cools, it contracts morethanthe iron.or steel of the rotor-core. On account of the. permanent set sufferedduring. the previous. starts, each stop causes the copper to draw in a.small fractional percentage of its length,,behindits previous condition;and this keeps up until a ground, or shortcircuit is produced. at the.place. where, the endturn portion of, the. winding bends. away from thestraight coil-sideportion.v This difficulty has been so acute that ithas been seriously proposed, both before and after the Rose invention,particularly in connection with machines not using the Rose invention,to attempt the difficult-task of preheating the turbo-rotor beforeputting, the machine in operation, after eachperiod of non-operation.

Ithas long been. known. that. copper, in. common with other metals,exhibits a phenomenon known as creep, which is a plastic strain ordeformation which increases slowly with time, when the copper is underany stress of either compression or tension, the creep-rate increasingwith the temperature ofthe copper. Ithasalso been known, for a number ofyears, that this creep-phenomenon is present in turbo generator rotors,and that it partially nullifies the benefits, of a small amount ofcold-working, depending upon the length of time the machine is operatedbetween its stop}- ping-periods. It has also been known, for anunrber ofyears, that an extremely small amount: of silver, alloyed with thecopper, will materially decrease its creep-rate, although it has notbeen known how much silver should" be used, as the reduction in creeprate, with the addition oi: silver, is not a continuous process; butthere is an optimum amount of silver, which; if exceeded, will result inbeginning to increase the creep-rate; instead of furtherreducing it.

Much of the copper in commercial use is derived from the melting ofconcentrates, and nearly all of the copper thus derived contains silverin vary,- ing amounts. The American Society for Testing Materials hasestablished specifications for copper for electrical purposes, requiringthat the metal contain 99.90% copper (plus silver). The aver:- agequality of commercial grades of electrical copper is much better thanthis. Coppers, containing various small amounts of silver, within therange in which my present invention is concerned. have long been known,and available on the market, and their electrical and mechanicalproperties have been extensively investigated, except in the matter ofthe creep-rate, which requires extended tests, extending over longperiods of time.

The prior knowledge on the subject may be roughly summarized as follows.In r944, C. M; Laffoon predicted that the use of some unnamed alloy ofcopper will improve the resistance of copper to creep in the rotors ofturbine generators. In the same year, P. H. Brace published some early,unrevised findings of the present applicant, showing that a decreasedcreep-rate is obtainable by the addition of 0.078% of silver tosubstantially pure copper, corresponding to 25 avoirdupois ounces ofsilver per ton. Some years previously, in 1932, H. J. Tapsell and A. E.Johnson indicated that silver-bearing arsenical copper (specifically.072% silver), particularly after 51% cold-work, is more resistant tocreep than arsenical copper without silver.

It is the object of my invention to build turbogenerator rotor-windingswith copper containing preferably more silver than 0:078:95, or-25avoi-rdupois ounces of silver per ton oi copper, but pref erably notmore than 0.312%, or I00 avoirdupois ounces of silver per ton, with thepartial 0016'.- working substantially like that which. was recommendedby Rose for substantially pure copper;

An exemplary turbine generator embodying invention is shown in theaccompanying drawing, wherein:

Figure 1 is a longitudinal sectional view of. a-

3 portion of a turbo-generator embodying my invention; and

Fig. 2 is a perspective view of an end of a. rotor member, with certainparts removed, the omitted parts including the retaining ring, the endplate and the blower.

The illustrated turbine generator comprises a stator member H, and arotor member i2. The stator member comprises a stator-core l3 carrying athree-phase winding I4.

The rotor member l2 comprises a drum-type. slotted rotor-core itcarrying a two-pole fieldwinding 20) having straight coil-sides 2| lyingin rotor-slots 22, which are closed by slot-wedges 21. The rotor-core isat least four feet long, in machines to which my invention isapplicable. The Helm-winding also has end-turns 28, which extend outbeyond the rotor-core l6. It is at the bends in the end-turn portions28, where they join onto the straight coil-side portions 2|, where thediiiiculty has been experienced because of the cumulative contractionsof the length of the straight coil-side portions 2|.

Sometimes, as pointed out in the Rose patent, a turbine generator willhave four poles, though usually it has two poles; and these generatorsgenerally operate at a primary-winding frequency of 50 to 60 cycles.

In accordance with my present invention, the conducting material of thefield-windings 2G, or at least the straight coil-sides 2! which lie inthe slots 22, are of argentiferous copper having more than 25 and lessthan 100 avoirdupois ounces of silver per ton. It is desirable to havean amount of silver which is somewhere near to a close approximation ofthe optimum amount from the standpoint of producing the smallest amountof creep, or the smallest creep-rate. When the copper is subjected to astress, it begins to creep, at a rate which is somewhat faster at first,but gradually tapers off, until, after a month or two, the creep-ratebecomes a constant amount which will be the minimum creep-rate for anyparticular stress at any particular temperature. It is obviouslydesirable to utilize an amount of silver which will give a sumcientlyclose approximation to the minimum creep-rate at the temperature andstress conditions which prevail in the turbogenerator rotor.

The minimum creep-rate is obtained, at the operating-conditions in aturbo-generator rotor;

when silver is present in the copper in an amount approximating 5Oavoirdupois ounces of silver per ton; or a little more than this figure,possibly something like from 56 to 64 ounces, or say 60 ounces, may be amore accurate optimum figure, although the precise amount of silver isnot too critical, because the curve of the minimum creeprate plottedagainst silver-content is substantially constant at the lowest minimumcreep-rate portion of the curve. Silver-amounts approximating 50avoirdupois ounces, within the limits plus or minus 40%, per ton, areacceptable; or between 30 and 90 avoirdupois ounces of silver per ton;or between 50 and 75 avoirdupois ounces of silver per ton; dependingupon the amount of departure from the absolute optimum conditions whichis to be tolerated, and also depending upon the operating-temperatureswhich are going to be maintained.

Before the field-windings 2!! are placed in the slots of the rotor-core,and in fact before the coils of the field-winding are even wound orformed, the copper straps which are used in the ileld-Winding arecold-Worked so as to cause a 6% reduction in the cross-section, or sayfrom 4% to 10% reduction, precisely as described, in the Rose patent,for the substantially pure copper straps, and for the same reason,namely, to increase the yield point to a value higher than thecompressional stress to which the winding will be subjected by reason ofthe thermal expansions in the rotor-core, without, however, increasingthe yield point to such a high value as to interfere with the bendingand/or straightening of the conductor in the process of forming thecoils.

During the use of a turbo-generator having an argentiferous-copperrotor-winding, in accordance with my invention, the rotor-copper and therotor-iron usually reach a certain temperature which is somewhat abovethe room-temperature, during each shut-down period when the machine isnot in operation. Each time the machine is started up, it is firstbrought up to speed, which imposes high centrifugal forces on the:roton-windings, pressing the straight coil-sides it hard against theretaining-wedges 21 of the slots, these being sufilcient to develop aconsiderable frictional force resisting any subsequent expansion of thestraight coil-sides, due to heating. After the machine is brought up tospeed and synchronized, load is applied, the rotor begins to heat up,and in due course it reaches its steady-state temperature, which isusually of the order of C. for the iron or steel of the rotor-core I6,and C. for the copper of the coil-sides 2|. Since the thermalcoefiicient of expansion of the copper is higher than that of the iron,and since the temperature of the copper is also higher, the copper tendsto expand, but the friction developed by the centrifugal force is sogreat that the central portions of the copper, in the middle of thestraight coil-side portions, cannot expand.

In the middle of each coil-side portion 2|, the copper thus compressesin an amount equal to the difference in thermal expansion between copperand steel, developing a compressive stress S represented by theequation,

where l4.5 10 =the modulus of elasticity of copper in pounds persquare-inch,

l7 l0 the thermal coefiicient of expansion of copper per C.,

11 10 =the thermal coefficient of expansion of steel,

Tc=the final temperature of the copper,

Ti=the final temperature of the iron, and

T0=the initial or cold temperature.

When the starting temperature To=50, the copper-temperature Tc=l25, andthe iron-temperatures Ti=85,

(2) S=12,900 pounds per square-dnch, which is a fair estimate ofoperating-conditions at present practices in regard to temperatures.

At points away from the middle of the rotorcore, that is, near the endsof the straight coilside portions 2|, the frictional forces resultingfrom the centrifugal force pressing the coil-sides out against theretaining-wedges diminish, reaching zero at the extreme ends of thecoils. At any distance at from each end of a coil-side, or from each endof the wedge, the frictionally imposed stress, which is operative on thecopper by reason of the internal portion of the copper trying to expandthermally, will be represented by the equation where p=centrifugal forcof a unit length of coil-side, f=friction coefficient, and w=distancefrom end of coil.

This formula for the frictionally developed compressive force S holdstrue for a distance a: in from each end of the coil, until S becomesequal to S, after which, the compressive stress on the copper remainsconstant at S for all larger values of :13, that is, for the internalportions of the coilside, as previously explained.

The effect of the addition of an optimum amount of silver tosubstantially pure copper, for the rotor-conductors of my invention, isto materially reduce the creep-rate of the copper, that is, the verysmall fraction of the total length of the coil-side by which it creeps,or releases, or plastically or permanently compresses during each hourwhen it is being subjected to a compressive stress at its operativetemperature. As the copper creeps, it relaxes the thermal compressivestresses to which it is subjected, so that, in a few months, say in ayears continuous operation of the turbo-generator without a shutdown,the relaxation of the compressive stress S operating on the centralportions of each coilside of my argentiferous-copper rotor-winding willreduce the stress to something like or /2 of the original value S whichis shown in Equations 1 or 2. With the pure-copper windings of the Rosepatent, the efiect of relaxation due to creepage reduced the compressivestresses much more.

It should be noted that the rate of relaxation is the most rapid atfirst, and this rate slows down in the later stages. Thus, in a machinefor which there are many startings and stoppings, the accumulatedrelaxation is greater than in a machine which runs continuously. Thepermanent shorting of a coil, upon unloading, that is, during eachstopping of the machine, is proportional to the amount by which thestress relaxed while under compression.

The addition of 50 avoirdupois ounces of silver per ton, to pure copperor electrolytic copper, makes the creep-rate 5 to times slower, atpresent operating temperatures, thus making a very material increase inthe number of years during which the turbine generator can be keptoperative without encountering a failure due to shortening of the rotorcoil-sides.

I claim as my invention:

1. In a turbo-generator having a stator member and a drum-type, slotted,iron rotor member having slots of at least 4 feet in length, thecombination, with said rotor member, of a winding having coil-sideslying in the rotor-slots, at least substantially all of those portionsof the winding which lie in said slots being composed of argentiferouscopper which has been coldworked so as to reduce its cross-sectionbetween 4 and 10 per cent, said argentiferous copper having more than 25and less than 100 avoirdupois ounces of silver per ton.

2. In a turbo-generator having a stator member and a drum-type, slotted,iron rotor member having slots of at least 4 feet in length, thecombination, with said rotor member, of a winding having coil-sideslying in the rotor-slots, at least substantially all of those portionsof the winding which lie in said slots being composed of argentiferouscopper which has been coldworked so as to reduce its cross-sectionbetween 4 and 10 per cent, said argentiferous copper havingapproximately 50 avoirdupois ounces of silver, within the limits plus orminus 40 per cent, per ton.

3. In a turbo-generator having a stator member and a drum-type, slotted,iron rotor member having slots of at least 4 feet in length, thecombination, with said rotor member, of a winding having coil-sideslying in the rotor-slots, at least substantially all of those portionsof the winding which lie in said slots being composed of argentiferouscopper having betwen 30 and avoirdupois ounces of silver per ton.

4. In a turbo-generator having a stator member and a drum-type, slotted,iron rotor member having slots of at least 4 feet in length, thecombination, with said rotor member, of a winding having coil-sideslying in the rotor-slots, at least substantially all of those portionsof the winding which lie in said slots being composed of argentiferouscopper having between 50 and '70 avoirdupois ounces of silver per ton.

EVAN A. DAVIS.

REFERENCES CITED The following referenlces are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,028,985 Behrend June 11, 19022,103,795 Rose Dec. 28, 1937 OTHER REFERENCES AIEE Transactions, 1944,page 1353. Journal IEE, April 1946, page 209. Journal IEE, April 1946,pages 210-211. Journal IEE, April 1946, page 212.

